Quadrupole mass spectrometer

ABSTRACT

A quadrupole mass spectrometer includes means for measuring total ion current having an electron repeller cage disposed about an electron source filament. The electron repeller cage repels electrons emitted by the electron source filament, urging them away from both the repeller and the filament, while also attracting positive ions. When the positive ions contact the electron repeller cage, a current is induced that is measured at the electron repeller cage. The measured current represents total ion current transmitted through the spectrometer. Thus, the need to include a total pressure measurement plate to measure total ion current is eliminated, and since the ion-exposed surface area of the electron repeller cage is greater than the ion-exposed surface area of the total pressure measurement plate, the invention provides improved total pressure measurement sensitivity.

FIELD OF THE INVENTION

This invention relates generally to mass spectrometers, and moreparticularly to quadrupole mass spectrometers.

BACKGROUND OF THE INVENTION

Quadrupole mass spectrometers are known. A portion of a prior artquadrupole mass spectrometer is shown in FIG. 1 of the drawings. Aground plate 10 supports an electron source filament 12 via twoconductive posts 14. The electron source filament 12 carries a currentof sufficient magnitude so that electrons having a negative charge areemitted by the electron source filament 12, i.e., the electron sourcefilament 12 serves as a cathode. The electrons are acceleratedelectrostatically towards an ion source cage 16 by an electric field inthe region between an electron repeller cage 18 and the ion source cage16. Positive ions are produced inside the ion source cage 16 when theaccelerated electrons strike neutral gas particles, in the form of atomsand molecules or a mixture thereof, within the cage along the path ofthe electrons. The positive ions within the ion source cage 16 areaccelerated towards a quadrupole mass filter 20 by a focus plate 30. Thequadrupole mass filter 20 includes a quadrilaterally symmetric parallelarray of four rods 22. Prior to entering the interior of the quadrupolemass filter 20, the positive ions pass through an aperture 24 of a totalpressure measurement plate 26 described in greater detail below.

To obtain an indication of the mass spectrum of the ions in the spacedefined by the ion source cage 16, a constant (DC) and superimposedsinusoidally modulated (RF) voltage is applied to the rods 22 of thequadrupole mass filter 20, and are scanned in tandem such that theirratio remains constant. More specifically, each diametrically oppositepair of rods are connected together. A signal (U+Vcosωt), which includesa positive DC component (U) and a radio frequency (RF) component(Vcosωt), is applied to one pair of rods, while a signal (-U-Vcosωt),which includes a negative DC component (-U) and a radio frequency (RF)component (-Vcosωt) opposite in phase (180°) to the RF component of thefirst mentioned signal, is applied to other pair of rods. The DC and RFcomponent signals are scanned such that their ratio of amplitudes, U/V,is kept constant. The fraction of the total ion current that exits thequadrupole mass filter 20 is partitioned according to the mass-to-chargeratio of each ion of the ion current. By scanning the RF voltagecomponent from a low to a high value, a plurality of ions, each having aparticular mass-to-charge ratio and arriving simultaneously at theentrance to the quadrupole mass filter 20, will arrive sequentially andordered according to mass-to-charge ratio at the exit of the quadrupolemass filter 20. Generally, by scanning the RF voltage component from alow to a high value, ions having a relatively low mass-to-charge ratiowill arrive at the end of the quadrupole mass filter 20 before ionshaving a relatively high mass-to-charge ratio. The ion current exitingthe filter 20 is sensed by a detector (not shown), such as a Farradaycup.

In the prior art device described with respect to FIG. 1, the totalpressure plate 26 is used to measure the total pressure of the gaswithin the device. Since the aperture 24 of the total pressuremeasurement plate 26 is smaller than the aperture 28 of the focus plate30, a known fraction of the total ion current provided to the filter 20is collected by the total pressure measurement plate 26. A currentmeasurement device 29 connected to the pressure measurement plate 26then provides a current signal as a function of the ion currentcollected by the total pressure measurement plate 26, and is thereforerepresentative of the total ion current entering the quadrupole massfilter 20. Once the total ion current is known, the total pressure P_(T)is obtained by multiplying the ion current by an empirically determinedconstant multiplicative factor. However, error can be introduced intothis measurement of total pressure P_(T) due to fringe field effects atthe entrance of the quadrupole mass filter 20, such as "reflection" ofions back towards the plate 26 after they pass through the aperture 24."Reflection" is a deflection of the original trajectory of the ions dueto repulsive forces resulting from the fringe fields.

Accordingly, another known method of measuring total pressure P_(T) hasbeen developed that allows for the elimination of total pressure plate26, thereby eliminating the problem created by the fringe field effectat the entrance of the quadrupole mass filter 20. In this prior artmethod, the DC voltage applied to the quadrupole mass filter 20 is setto 0 volts, and all of the ions that pass through the focus plate 30will enter the quadrupole mass filter 20, and will be collected andmeasured at a detector (not shown in FIG. 1) as they exit the quadrupolemass filter 20. The ion current that is measured at the detector as theions exit the quadrupole mass filter 20 represents the total ioncurrent, from which can be calculated the total pressure P_(T). However,in practice, inaccuracies can arise due to lighter ions, such ashydrogen and helium gas ions, failing to arrive at the detector (notshown) at the exit of the quadrupole mass filter 20. Consequently, thevalue obtained for total pressure P_(T) is significantly inaccurate.

OBJECTS OF THE INVENTION

It is a general object of the present invention to provide a quadrupolemass spectrometer of the type described that significantly overcomes theproblems of the prior art.

A more specific object of the present invention is to provide aquadrupole mass spectrometer having a simplified ion source.

Another object of the invention is to provide a quadrupole massspectrometer that overcomes the problem of ion reflection at quadrupolefringe fields adversely affecting total pressure measurement.

Another object of the invention is to provide a quadrupole massspectrometer that exhibits increased sensitivity of partial pressuremeasurements.

Another object of the invention is to provide a quadrupole massspectrometer that exhibits significantly increased measured total ioncurrent.

Another object of the invention is to provide a quadrupole massspectrometer that maximizes the ion current transmitted through thequadrupole of the mass spectrometer.

Another object of the invention is to provide a quadrupole massspectrometer that maximizes the stability of the measured total ioncurrent.

Other objects of the invention will in part be obvious and will in partappear hereinafter. The invention accordingly comprises the apparatuspossessing the construction, combination of elements, and arrangement ofparts which are exemplified in the following detailed disclosure, andthe scope of the application of which will be indicated in the claims.

SUMMARY OF THE INVENTION

A quadrupole mass spectrometer is provided that includes means formeasuring total ion current collected by an electron repeller cagedisposed about an electron source filament. The electron repeller cagerepels electrons emitted by the electron source filament, urging themaway from both the repeller and the filament, while also attractingpositive ions that induce a measurable current in the electron repellercage upon contact therewith. The current measured at the electronrepeller cage is proportional to the total ion current transmittedthrough the spectrometer. Thus, the need to include a total pressuremeasurement plate to measure total ion current is eliminated, withoutthe disadvantages associated with setting the DC voltage in the rods ofthe quadrupole mass filter to zero. Additionally, since the ion-exposedsurface area of the electron repeller cage is greater than theion-exposed surface area of the typical prior art total pressuremeasurement plate, the invention provides improved total pressuremeasurement sensitivity. Further, by attracting ions created outside theion source cage to the electron repeller, as opposed to using a part ofthe ion current applied to the quadrupole filter, the measurement oftotal pressure has little affect on the ion current applied to thefilter.

As is generally known, the electron repeller cage assists the ion sourcecage in urging electrons emitted from the electron source filament intothe ion source cage. See, for example, U.S. Pat. No. 4,579,144(Kuo-Chin, el. al.) and U.S. Pat. No. 4,689,574 (Kuo-Chin, et al.).However, in accordance with the present invention, it has beenappreciated that the emitted electrons strike neutrally charged gasparticles, in the form of atoms and molecules or a mixture thereof,disposed both outside and within the ion source cage, yielding positiveions. By connecting the electron repeller at an appropriate potential,preferably system ground, the electron repeller can attract positiveions residing outside the ion source cage. Within the ion source cage, afocus electrode accelerates the positive ions disposed therein into aquadrupole mass filter, without passing through a total pressuremeasurement plate, because total pressure is obtained from themeasurement of the current at the electron repeller cage.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood from the following detaileddescription, in conjunction with the accompanying figures, wherein:

FIG. 1 is an exploded perspective view of the ion source assembly, focusplate, and a portion of the quadrupole mass filter of a prior artquadrupole mass spectrometer, the ion source assembly having a totalpressure measurement plate;

FIG. 2 is a schematic cut-away radial side view of the quadrupole massspectrometer of the present invention;

FIG. 3 is an exploded perspective view of the ion source assembly, focusplate, and a portion of the quadrupole mass filter of the quadrupolemass spectrometer of the present invention;

FIG. 4 is a schematic perspective view of the quadrupole massspectrometer of the present invention, including an ion detectorassembly;

FIG. 5A is a top view of an embodiment of an electron repeller cage ofthe type used in the present invention, disposed on a mounting plate;

FIG. 5B is a side view of the embodiment of FIG. 5A; and

FIG. 6 is a pictorial representation of the quadrupole mass spectrometerof the present invention mounted upon a vacuum flange.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

By way of further background, within the confines of a quadrupole massspectrometer, the total pressure P_(T) of a gas of neutral or ionizedparticles, where the particles can be either atoms, molecules, or amixture thereof, consists of the sum of the partial pressures P_(N) ofeach of the different constituents or trace elements of the gas. Statedmathematically,

    P.sub.T =P.sub.1 +P.sub.2 + . . . +P.sub.N.                (1)

For example, a gas consisting essentially of diatomic hydrogen H₂ andhelium He would have a total pressure equal to the sum of the partialpressure of diatomic hydrogen and the partial pressure of helium.Knowledge of total and partial pressures is useful for detecting leaksin a vacuum system, for example. For this and other reasons, it ishighly desirable to measure both total and partial pressures asaccurately and precisely as possible.

Both total and partial pressures are proportional to the correspondingvolumetric number density. Thus, it follows that the total volumetricnumber density N_(T), given as a number of atoms or molecules per cubiccentimeter, for example, is equal to the sum of the partial volumetricnumber densities of the constituents or trace elements. Statedmathematically,

    N.sub.T =N.sub.1 +N.sub.2 + . . . N.sub.N.                 (2)

The probability of an electron colliding with a neutral atom ormolecule, and thereby creating a positive ion is proportional to thevolume number density of the neutral atom or molecule along the electronflight path. Thus, the current induced when positively ionized particlescontact a surface instrumented with a current measurement device isproportional to the total volumetric number density of the neutral atomsor molecules, and therefore is proportional to the total pressure P_(T)of the neutral atoms or molecules. Thus, knowledge of the total currentdue to ions contacting the surface instrumented with the currentmeasurement device provides knowledge of the total pressure P_(T).

Referring to FIGS. 2, 3, and 4, a current-carrying electron sourcefilament 12 emits a plurality of electrons. An electron repeller cage 18is biased at a lower voltage than the filament 12. For example, theelectron repeller cage is biased at 0 volts by being electricallyconnected to a system ground of the mass spectrometer, and the filamentis biased at +125 volts. Also, the ion source cage 16 is biased at ahigher voltage than the filament, e.g., at +205 volts. Consequently,electrons emitted by the filament 12 are accelerated away from theelectron repeller cage 18, while positive ions resulting from electroncollisions with a neutral constituent or trace element are attractedtowards the electron repeller cage 18. When the positive ions contactthe electron repeller cage, a measurable current is induced therein,which current is measured by a current measurement device 21. Thecurrent measurement device 21 can measure currents in the range of 10⁻¹³amps to 10⁻⁷ amps. The current measured by the device 21 is proportionalto the total ion current.

The constant of proportionality that relates the measured current at theelectron repeller cage and the total ion current is a function of gasspecies or constituent, and electron energy, and has a typical value of3×10⁻⁴ amps/torr, for example. This value is derived empirically. Itmust be calibrated at the factory, and then is preferably recalibratedperiodically at a user site.

Preferably, a source cage 16 is disposed concentrically within theconfines of both the electron source filament 12 and the electronrepeller cage 18. Electrons emitted by the filament 12 that enter thevolume embraced by the source cage 16 generate positive ions 19 therein.The positive ions 19 are drawn towards the quadrupole mass filter 20 bya focus plate 30 that is biased at +180 volts, for example. The positiveions 19 pass through an aperture 28 of the focus plate 30 and then enterthe quadrupole mass filter 20, without being partially intercepted by atotal pressure measurement plate, as in the prior art. Thus, unlike theprior art, substantially all of the ions 19 that traverse the focusplate 30 enter the quadrupole mass filter 20 for detection at a partialpressure detector 32 disposed at the distal end of the mass filter 20.Ion currents are measured at the detector by a current measurementdevice 33. To correct for fringe field effects at the distal end of themass filter 20, a field correction plate 34 is preferably interposedbetween the partial pressure detector 32 and the mass filter 20.

RF and DC voltages applied to the quadrupole mass filter 20 are scannedto provide a so-termed mass spectrum, wherein ions characterized by arelatively high mass-to-charge ratio arrive at the detector 32 when theRF voltages are high, and ions characterized by a relatively lowmass-to-charge ratio arrive at the detector 32 when the RF voltages arelow. Since the applied RF voltage V_(RF) is proportional to the mass ofions collected at the detector 32, the ion current measured by themeasurement device 33 at the detector 32 is plotted as a function of theapplied RF voltage to provide a mass spectrum of the ions 19 generatedin the ion source cage 16.

In particular, the applied RF voltage V_(RF) is equal to the product ofthe mass `m` of a particular species or constituent in atomic massunits, the square of the frequency `f` of the applied RF voltage inmegahertz, the square of the inscribed radius r₀ (in centimeters) of thequadrupole mass filter 20, and the constant value 7.219. Statedmathematically,

    V.sub.RF =7.219*m*r.sub.0.sup.2 *f.sup.2.                  (3)

Referring to FIGS. 5A and 5B, the electron repeller cage 18 includes amesh region 40 having a property of both allowing high transit rate ofneutral gas particles into its interior region 42, and supporting anelectric field of a strength sufficient to repel electrons and attractpositive ions. A preferred open area coefficient is about 60%, i.e., 60%of the surface area of the electron repeller cage 18 should permit freepassage of neutral gas particles residing in a region 44 outside thecage 18 to the interior region 42 of the cage 18, although this areacoefficient can vary. Electrical connection of a current measurementdevice is accomplished using a conductive post 46. The cage 18 ismounted upon and electrically coupled to an electron repeller supportplate 48.

With reference to FIG. 6, the entire quadrupole mass spectrometer 50 ismounted upon a vacuum flange 52, and is thereby enclosed within a vacuumchamber capable of maintaining a vacuum of less than 10⁻⁴ torr, forexample. The electron repeller cage 18 is to be recognized at the top ofthe figure. The ion source cage 16 (not shown in FIG. 6) is supported byan ion source cage support plate 54. The ion source cage 16 resideswithin, and is therefore mostly obscured by, the electron repeller cage18 in this figure. Thus, the quadrupole mass spectrometer described inFIGS. 2-6 provides an improvement over the prior art.

Specifically, the quadrupole mass spectrometer of the invention includesa simplified ion source in that it does not require a total pressuremeasurement plate. Further, since a total pressure measurement plate isnot used, the problem of ion reflection at quadrupole fringe fields isavoided. Moreover, due to the superior collection of ion current at theelectron repeller cage sensitivity of total pressure measurements isincreased, and the stability of the measured ion current is maximized.Additionally, since the invention does not depend upon a total pressuremeasurement plate, substantially all of the ions produced in the ionsource cage enter the quadrupole mass filter, thereby providing aquadrupole mass spectrometer that exhibits significantly increasedtransmission through the quadrupole mass filter. This results in moresensitive measurements of partial pressures. Also, since the surfacearea of the electron repeller cage that is exposed to ions generatedoutside the ion source cage is significantly greater than the surfacearea of the pressure measurement plate that is exposed to ions generatedinside the ion source cage, a larger total ion current signal isobtained by instrumenting the electron repeller cage for measuring totalion current, omitting the pressure measurement plate entirely. Further,the ion current provided to the mass quadrupole filter is derived frompositive ions created in the ion source cage, while the total pressurecurrent provided off of the electron repeller is derived from positiveions created outside the ion source cage. Thus, one will have no affecton the other. Additionally, since the method of measuring the total ioncurrent does not involve the mass filter at all, problems in the priorart relating to the failure to trap light ions are entirely avoided.

Other modifications and implementations will occur to those skilled inthe art without departing from the spirit and the scope of the inventionas claimed. Accordingly, the above description is not intended to limitthe invention except as indicated in the following claims.

Since certain changes may be made in the above apparatus withoutdeparting from the scope of the invention herein involved, it isintended that all matter contained in the above description or shown inthe accompanying drawing shall be interpreted in an illustrative and notin a limiting sense.

What is claimed is:
 1. A quadrupole mass spectrometer for measuring therelative amounts of the respective constituents of a gas present in saidspectrometer, said spectrometer being of the type comprising aquadrupole mass filter, the quadrupole mass spectrometercomprising:means for defining a space for containing a representativesample of said gas; biased electron repeller means for repellingelectrons and attracting positive ions contained within said space;electron source means, disposed generally within said space, foremitting a plurality of electrons, the plurality of electrons beingaccelerated into said space so as to produce a plurality of positiveions, a fraction of the plurality of positive ions being attracted tothe electron repeller means; and current measurement means, coupled tothe electron repeller means, for measuring a current that results fromthe fraction of the plurality of positive ions that are attracted to theelectron repeller means, and for providing a signal representative ofthe total ion current entering the quadrupole mass filter.
 2. Aquadrupole mass spectrometer according to claim 1, further including ionsource means, disposed in said space, for providing a second fraction ofthe plurality of positive ions of said constituents of said gas and forgenerating the ion current entering said quadrupole mass filter.
 3. Aquadrupole mass spectrometer according to claim 2, wherein said ionsource means comprises an ion source cage.
 4. A quadrupole massspectrometer according to claim 1, wherein said electron source meansincludes a filament.
 5. A quadrupole mass spectrometer according toclaim . .1.!. .Iadd.9.Iaddend., wherein electron source means forproducing electrons comprises a filament disposed in said space.
 6. Aquadrupole mass spectrometer according to claim . .1.!..Iadd.9.Iaddend., wherein said ion source means comprises a ion sourcecage.
 7. A quadrupole mass spectrometer according to claim . .1.!..Iadd.9.Iaddend., wherein said means for generating an ion current fromthe ions produced inside said ion source means so that said ion currentrepresents the relative amounts of said constituents comprises aquadrupole mass filter.
 8. A quadrupole mass spectrometer according toclaim . .1.!. .Iadd.9.Iaddend., wherein said means for propelling saidelectrons produced by said electron source means through said space intosaid ion source means so that positive ions of each said constituent areproduced in said space includes means for biasing said electron repellermeans relative to said electron source means and said ion source means.9. A quadrupole mass spectrometer for measuring the relative amounts ofmass of one or more constituents, respectively of different molecularweights, of a gas, said spectrometer comprising:(a) means for defining aspace for containing a representative sample of said gas; (b) electronsource means for producing electrons; (c) ion source means, disposed insaid space, for producing ions of each of said constituents of said gaswhen particles of each constituent present in said ion source means arestruck by electrons; (d) means for propelling said electrons produced bysaid electron source means through said space into said ion source meansso that positive ions of each said constituent are produced in saidspace, both inside and outside said ion source means, said means forpropelling said electrons including electron repeller means forrepelling said electrons toward said ion source means and for collectingions received from said space; and (e) means for generating an ioncurrent from the ions produced inside said ion source means so that saidion current represents the relative amounts of said constituents; and(f) means for generating a signal as a function of the ions collected bysaid electron repeller means and representative of the total ion currentproduced inside said ion source means.
 10. In a quadrupole massspectrometer comprising an electron source filament, the electron sourcefilament carrying a current of sufficient magnitude such that electronshaving a negative charge are emitted by the electron source filament,the electrons being accelerated towards an ion source cage by anelectric field in the region between an electron repeller cage and theion source cage, positive ions being produced both inside and outsidethe ion source cage when the accelerated electrons strike neutral gasparticles along the path of the electrons, the positive ions within theion source cage being accelerated towards a quadrupole mass filter by afocus plate, the quadrupole mass filter including a quadrilaterallysymmetric parallel array of four charge-balanced rods, the improvementcomprising:current measurement means, coupled to the electron repellercage for providing a signal representative of the total ion currententering the quadrupole mass filter. .Iadd.
 11. A mass spectrometer formeasuring the relative amount of the respective constituents of a gaspresent in said spectrometer and including a gas analyzer, the massspectrometer comprising:means for defining a space such that arepresentative sample of a gas present in said spectrometer is presentinside and outside of said space: electron source means, disposedoutside said space, for emitting a plurality of electrons, the pluralityof electrons being accelerated towards said space so as to produce aplurality of positive ions; means for generating a first ion current anda second ion current from said plurality of positive ions, said firstion current tending to travel in a first direction towards said gasanalyzer, and said second ion current tending to travel in a seconddirection different from said first direction; and current measurementmeans for measuring said second ion current..Iaddend..Iadd.12. A massspectrometer according to claim 11, wherein said first direction issubstantially perpendicular to said second direction..Iaddend..Iadd.13.A mass spectrometer according to claim 11, wherein said second ioncurrent is representative of said first ion current..Iaddend..Iadd.14. Amass spectrometer according to claim 11, wherein said second ion currentis representative of a pressure of said gas..Iaddend..Iadd.15. A massspectrometer according to claim 11, wherein said gas analyzer comprisesa quadrupole mass filter..Iaddend..Iadd.16. A mass spectrometeraccording to claim 11, wherein said means for defining a space comprisesan ion source cage..Iaddend..Iadd.17. A mass spectrometer according toclaim 11, wherein said means for defining a space comprises a closed ionsource..Iaddend..Iadd.18. A mass spectrometer for measuring the relativeamounts of mass of one or more constituents of a gas, said spectrometercomprising:(a) means for defining a space such that a representativesample of a gas in the spectrometer is present inside and outside ofsaid space; (b) electron source means, disposed outside said space, forproducing electrons; (c) means for propelling said electrons produced bysaid electron source means into said space so that positive ions of eachsaid constituent are produced both inside and outside said space; (d)means for generating an ion current from the ions produced inside saidspace so that said ion current represents the relative amounts of saidconstituents inside said space; (e) ion collector means for collectingions generated outside said space; and (f) means for generating a signalas a function of the ions collected by said ion collector means andrepresentative of said ion current..Iaddend..Iadd.19. A massspectrometer according to claim 18, wherein said means for defining aspace comprises an ion source cage..Iaddend..Iadd.20. A massspectrometer according to claim 18, wherein said means for defining aspace comprises a closed ion source..Iaddend..Iadd.21. A massspectrometer according to claim 18, wherein said signal isrepresentative of a pressure of said gas..Iaddend..Iadd.22. A massspectrometer according to claim 18, further comprising gas analyzermeans for analyzing the ions generated inside saidspace..Iaddend..Iadd.23. A mass spectrometer according to claim 22,wherein said gas analyzer means comprises a quadrupole massfilter..Iaddend..Iadd.24. In a mass spectrometer comprising an electronsource filament, the electron source filament carrying a current ofsufficient magnitude such that electrons having a negative charge areemitted by the electron source filament, the electrons being acceleratedtowards a space by an electric field in the region between the electronsource and the space, positive ions being produced both inside andoutside the space when the accelerated electrons strike neutral gasparticles along the path of the electrons, the improvementcomprising:ion collector means for attracting a fraction of said ionsproduced outside the space and for generating therefrom a signalrepresentative of the number of ions produced inside thespace..Iaddend..Iadd.25. In a mass spectrometer according to claim 24,wherein said signal is representative of a pressure of a gas in saidspace..Iaddend..Iadd.26. A quadrupole mass spectrometer for measuringthe relative amounts of the respective constituents of a gas present insaid spectrometer, said spectrometer being of the type comprising aquadrupole mass filter, the quadrupole mass spectrometercomprising:means for defining a space for containing a representativesample of said gas; biased electron repeller means for repellingelectrons; collecting means within said space for attracting andcollecting positive ions contained within said space; electron sourcemeans disposed generally within said space for emitting a plurality ofelectrons, the plurality of electrons being accelerated into said spaceso as to produce a plurality of positive ions a fraction of theplurality of positive ions being attracted to the collecting means; andcurrent measurement means, coupled to the collecting means, formeasuring a current that results from the fraction of the plurality ofpositive ions that are collected by the collecting means, and forproviding a signal representative of the total ion current exiting saidspace and entering the quadrupole mass filter..Iaddend..Iadd.27. Aquadrupole mass spectrometer according to claim 26, further includingion source means, disposed in said space, for providing a secondfraction of the plurality of positive ions of said constituents of saidgas and for generating the ion current entering said quadrupole massfilter..Iaddend..Iadd.28. A quadrupole mass spectrometer according toclaim 27, wherein said ion source means comprises and ion sourcecage..Iaddend..Iadd.29. A quadrupole mass spectrometer according toclaim 26, wherein said electron source means includes afilament..Iaddend..Iadd.30. A quadrupole mass spectrometer for measuringthe relative amounts of mass of one or more constituents, respectively,of a gas, said spectrometer comprising: means for defining a space forcontaining a representative sample of said gas; electron source meansfor producing electrons; ion source means, disposed in said space, forproducing ions of each of said constituents of said gas when particlesof each constituent present in said ion source means are struck byelectrons; means for propelling said electrons produced by said electronsource means through said space into said ion source means so thatpositive ions of each said constituent are produced in said space, bothinside and outside said ion source means, said means for propelling saidelectrons including electron repeller means for repelling said electronstoward said ion source means; collecting means for collecting ionsreceived from said space and produced outside said ion source means;means for generating an ion current from the ions produced inside saidion source means so that said ion current represents the relativeamounts of said constituents; and means for generating a signal as afunction of the ions collected by said collecting means andrepresentative of the total ion current produced inside said ion sourcemeans..Iaddend..Iadd.31. A quadrupole mass spectrometer according toclaim 30, wherein the electron source means for producing electronscomprises a filament disposed in said space..Iaddend..Iadd.32. Aquadrupole mass spectrometer according to claim 30, wherein said ionsource means comprises an ion source cage..Iaddend..Iadd.33. Aquadrupole mass spectrometer according to claim 30, wherein said meansfor generating an ion current from the ions produced inside said ionsource means so that said ion current represents the relative amounts ofsaid constituents comprises a quadrupole mass filter..Iaddend..Iadd.34.A quadrupole mass spectrometer according to claim 30, wherein said meansfor propelling said electrons produced by said electron source meansthrough said space into said ion source means so that positive ions ofeach said constituent are produced in said space includes means forbiasing said electron repeller means relative to said electron sourcemeans and said ion source means..Iaddend.